[discord] migrate to unique usernames

[iamtraction]

• [x] Remove any logic that depends on discriminators for Discord users. Make sure to account for the temporary #0 discriminator that updated users will have when updating your code.
  • Discriminator is currently only used in Omnic & Dashboard. Remove it after Discord moves to unique usernames.
• [x] Prepare for the bot's username change (in code, or anywhere else the bot's username is referenced).
  • https://github.com/TheBastionBot/Bastion/blob/9c6d7dba9f3a2e8966fc2d8138fb97f87756361d/.github/SECURITY.md?plain=1#L14

Ref https://support-dev.discord.com/hc/en-us/articles/13667755828631

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